High frequency alternating current waveforms in the kilohertz range (KHFAC waveforms) can provide nerve block (e.g., to control pain and/or spasticity). For example, the nerve block can be immediate, partial, selective, complete, and/or fully reversible. However, many stimulators that can provide KHFAC waveforms are impractical or impossible to incorporate in an implantable neural prosthesis devices. These stimulators require a large, heavy battery that is impractical for an implantable device. Additionally, if any portion of the stimulator were to experience an electric failure, such as open circuit or a short circuit, the stimulator could provide a continuous supply of direct currents (DC) to the stimulation electrode of the neural prosthesis, potentially damaging the electrode and/or the nerve.